Currently SRFI 4 specifies that make-TAGvector when not given a
fill value will fill the homogeneous vector with zero:
For the procedure make-TAGvector the fill value
defaults to exact 0 for integer vector datatypes and inexact 0 for
the float vector datatypes.
This departs from the make-string and make-vector procedures which leave
the content unspecified. I think the same principle should apply to
make-TAGvector because the semantics will be closer to make-string and
make-vector, and also for efficiency, because with homogeneous vectors
the initialization of the elements can easily be avoided (it is possible
but much harder to do this for make-vector).